Glossy or shiny surfaces have always been desirable for appearance parts made of plastic. Glossy parts, for example the finish on a new automobile, are aesthetically appealing. Plastic manufacturers are continually striving for higher gloss surfaces to make their plastic parts more appealing and to open new markets for their plastics.
This is especially important in the sheet extrusion/thermoforming market because most thermoforming tools are designed such that, in operation, the tool does not touch the appearance side of the sheet. In injection molding, the mold can be made to have a very high polish. By using high pressure to force the molten plastic up against the mold, a high gloss surface is achieved, even though the material itself is not inherently high gloss.
This, however, is not normally the case when a sheet of plastic is thermoformed. The thermoforming process typically consists of stretching sheet materials in all directions. Pressures are much lower in thermoforming and the tools are generally much-lower cost than injection molds and are not designed for aesthetics. Thus, the inherent gloss of the plastic material itself is the determining factor of the gloss of the finished part.
The gloss of a surface is determined by the amount of light that is scattered when light hits the surface of the object. This scattering is a function of the roughness of the surface. Any discrete particles on the surface, especially those large enough to scatter light, affect the gloss. This is the reason that most ordinary particulate fillers when compounded into plastic reduce the gloss dramatically. Ordinary rubber particles which are used to toughen plastics, such as that found in early formulations of acrylonitrile-butadiene-styrene (ABS) and in block copolymer polypropylene (PP), have the same effect. It was well-known in the art that ABS manufacturers had problems with gloss because they could not make small enough rubber particles. Subsequent technology allowed for the manufacture of rubber domains small enough so that ABS could be made with much higher gloss. Nevertheless, it has always been difficult for makers of crystalline or semicrystalline resins to make a high gloss resin because the crystals themselves act like particulates and cause the surface to be rough. High density polyethylene (HDPE) has had this problem, and so has PP to a lesser extent.
When making a sheet, it is possible to make a very high gloss surface by using highly polished chill rolls to cool the sheet. However, this high gloss finish disappears whenever the plastic is stretched and the inherent gloss of the plastic material shows though.
Prior inventions have concentrated on the high gloss aspect of molding materials. For example, in U.S. Pat. No. 4,849,045, a high gloss surface appearance on a polyvinyl chloride (PVC) molding is attempted by removably laminating a smooth film to the surface of the molding piece immediately after extrusion and then stripping off the film after the molding piece is cooled to room temperature. However, this process has limited commercial applicability since only a single gloss level may be achieved by the use of a particular film having a fixed surface finish. A different gloss appearance would require a different type of film. Other efforts have been made to produce a high gloss stretchable sheet material, but a number of these efforts have resulted in coating defects and insufficient uniformity to even undergo the stresses of thermoforming.
As the search goes on for high gloss materials, the impact strength of the materials has generated less interest. However, the thermoforming industry has recognized the need for a high gloss, high impact, material to use for exterior parts. Certain known materials are insufficient for several reasons, which are quite evident. ABS, high impact polystyrene (HIPS), and polycarbonate (PC) cannot be UV-stabilized easily, if at all. HDPE is not glossy. Acrylic coextruded over ABS is brittle, especially at low temperatures. Ordinary high impact copolymer PP is difficult to thermoform and is not glossy. Filled CPP, while easier to thermoform, is not glossy. Homopolymer PP is brittle.
The present invention addresses these deficiencies of the prior art.